An air filter is an important component of an HVAC system for several reasons. For one, it acts to purify air in the environment at a site where the HVAC system is operating. For another, by removing dust and particulates from the air at a site, an air filter also contributes to the efficiency of the HVAC system. Over time, however, as dust and particulates accumulate and build up on the air filter, its efficacy is substantially diminished.
The effect of a dirty air filter in an HVAC system is essentially two-fold. Most noticeably, a dirty air filter restricts airflow through the ducting of an HVAC system. A consequence here is the reduction of airflow velocity in the air duct. Perhaps more importantly, however, in order to overcome the reduced airflow capability that is caused by a dirty air filter, the HVAC system must operate at higher power levels. The unwanted consequences of this are: 1) increased operational costs for the HVAC system, and 2) an increased possibility of a potentially unacceptable demand on the local power grid. Both are to be avoided.
As disclosed in U.S. Pat. No. 6,128,910 for an invention entitled “Diagnostic Unit for an Air Conditioning System”, which is assigned to Enalasys Corporation, the operational efficacy of an HVAC system can be determined by measuring enthalpies at selected locations in the system. Further, in U.S. application Ser. No. 15/428,643, filed on Feb. 9, 2017, for an invention entitled “Site-Customized Benchmark for Operating an Air Conditioning System in Real Time”, the operational efficacy of an HVAC system is based on enthalpy measurements that are responsive to environmental considerations at the site of the system. Suffice it to say here that, in both of these cases, enthalpy measurements are a function of the volume of air being measured. Accordingly, they are also a function of the velocity of airflow through an HVAC system. Thus, as recognized by the present invention, the overall efficiency of an HVAC system, and the efficacy of an air filter in the system are both related to a same airflow velocity v.
In addition to the considerations of enthalpy noted above, the present invention recognizes that decreases in air filter efficacy are a direct result of reductions to the velocity v of air flowing through the ducting of an HVAC system. Moreover, the present invention recognizes that in order to maintain the overall efficiency of an HVAC system, detrimental reductions in airflow velocity v need to be countered by increasing the operational power requirements for the HVAC system. Increased operational power requirements, however, have a practical limit. In any event, airflow velocity v is an important consideration.
It is well known that airflow velocities v can be measured by a pitot-static tube. In overview, a pitot-static tube functions on the principle that in any airflow, a total pressure=static pressure+dynamic pressure. In this relationship, the dynamic pressure=½ρv2, wherein ρ is air density and v is airflow velocity; and the static pressure is equal to the local atmospheric pressure. As noted above, in the context of the present invention, the airflow velocity v through an air duct is an important indicator of an air filter's efficacy.
In light of the above, it is an object of the present invention to provide a control unit which monitors the efficacy of an air filter in an HVAC system by evaluating the power requirements necessary for the system to maintain a constant airflow velocity through the air filter. Another object of the present invention is to provide a control unit which monitors the efficacy of an air filter in an HVAC system, and signals when the system's power requirements indicate that a replacement of the air filter is appropriate. Still another object of the present invention is to provide a control unit for monitoring the efficacy of an air filter in an HVAC system which is easy to install, is simple to use, and is cost effective.